There are many ways to transmit signals over conductors. In single-ended signaling, one conductor carries a signal as a voltage that varies over time. The signal is referenced to a fixed potential, which is usually a 0V node referred to as ground. Thus, one conductor carries the signal and one conductor carries the reference potential. The receiver extracts information by detecting the difference between the signal-carrying conductor and the reference potential.
In differential signaling (sometimes referred to as double-ended signaling), information is transmitted over two conductors using two complementary voltage signals, one over each conductor. One conductor carries the signal, and the other carries the inverted signal. The pair of conductors can be, for example, traces on a printed circuit board (PCB). The receiver extracts information from the pair of conductors by detecting the potential difference between the inverted and non-inverted signals. Ideally, the voltage signals on the two conductors have equal amplitude and opposite polarity relative to a common-mode voltage, in which case they are said to be “balanced.” The return currents associated with these voltages also have equal amplitude and opposite polarity and thus cancel each other out; for this reason, differential signals ideally have zero current flowing through the ground connection.
Relative to single-ended signaling, differential signaling offers a number of advantages for high-speed data transfer. For example, if electromagnetic interference (EMI; also referred to as radio-frequency interference (RFI)) or crosstalk (i.e., EMI generated by nearby signals) is introduced from a source outside the differential conductors, it is added equally to the inverted and non-inverted signal. Because the receiver operates on the difference in voltage between the two signals, the receiver circuitry will greatly reduce the amplitude of any interference or crosstalk that is present in the received signal. Thus, differential signals are less sensitive than single-ended signals to EMI, crosstalk, or any other noise that couples into both signals of the differential pair.
Another advantage of differential signaling is that because differential signals have higher EMI immunity than single-ended signals, differential signals can use lower voltages than single-ended signals and still maintain adequate signal-to-noise ratio (SNR). In addition, the SNR with differential signaling is two times that of an equivalent single-ended implementation because the dynamic range at the differential receiver is twice as high as the dynamic range of each signal within the differential pair. Several advantages flow from the ability of differential signaling to successfully transfer data using lower signal voltages, including that supply voltage requirements are lower, which reduces power consumption. In addition, smaller voltage transitions, which are possible because of greater immunity to EMI, allow for higher operating frequencies. Consequently, high-speed digital systems often use differential signaling.
Differential signaling also tends to cause less EMI than single-ended signaling. The rising and falling edges of digital signals can generate significant amounts of EMI, and both single-ended and differential signals generate EMI. But because the currents in the conductors in differential signaling travel in opposite directions, the two signals in a differential pair create electromagnetic fields that are opposite in polarity. If the differential signal paths are identical and in close proximity to each other, the individual electromagnetic fields caused by the two signals will largely cancel each other. If, however, the two signal paths are not identical, the generated magnetic fields will not be exactly equal and opposite and will not completely cancel each other. As a result, the common mode current on the two conductors is able to generate an electromagnetic field outside the pair of conductors, which act like an antenna and radiate EMI. In addition, due to integrated circuit process imperfections, mismatches in the different pair circuit drivers can produce an inherent common-mode signal, which can create EMI.
Although differential signal paths are ideally identical, and the signals carried on the two conductors ideally have equal amplitude and opposite polarity, practical systems using differential signaling typically suffer from intrinsic common-mode noise as well as interference caused by sources outside the differential conductors, and the differential conductors can also radiate EMI and thereby cause interference to external systems or nearby circuits. Collectively, the common-mode noise and interference (whether received or generated) are referred to herein simply as “common-mode noise.” Common-mode noise can be caused by clock skew, differences in amplitude between the signals on the two paths, unbalanced routing (e.g., one of the two paths is longer or shorter than the others, or the distance between conductors varies along their lengths, etc.), and other factors. Above the gigahertz frequency range, common-mode interference signals can degrade differential signal integrity and/or power integrity, and the use of differential signaling may also cause EMI. As a consequence, common-mode noise can degrade the SNR of the transmitted signal and cause detection errors. Likewise, single-ended signaling also suffers from noise and EMI, and can also generate EMI that can adversely affect other receivers.
Common-mode noise filters can be used to suppress common-mode noise and protect signal SNR. For example, the signal detection electronics can include a filter circuit, but the inclusion of such a filter circuit can increase the cost of the signal detection electronics. As another example, a common-mode filter can be mounted on the surface of a PCB through which the signal paths are routed. The use of one or more surface-mounted filters increases the cost of the populated PCB. As another example, a narrowband filter can be built into the PCB structure without added cost, but the bandwidth of this type of filter is usually narrow and can only target one frequency at a time. Because a single filter may not provide sufficient attenuation of the common-mode noise, or may not provide sufficient attenuation at all frequencies at which common-mode noise is problematic (e.g., when the common-mode interference signals have a signal at a base frequency and higher harmonics), it may be necessary to use multiple common-mode filters to attenuate the common-mode noise adequately at the frequencies where it is problematic. Even when a single filter should be sufficient, manufacturing tolerances can cause the filter frequency band to shift away from the target frequency, which can reduce the effectiveness of the filter. As a result, the need for one or more filters increases the size of the PCB or reduces the amount of PCB space available for other components.
Therefore, there is an ongoing need for alternative ways to reduce common-mode noise.